1. Field of the Invention
The present invention relates to a liquid-crystal projector. More particularly, the present invention relates to a liquid-crystal projector where an image projected on rear side of a screen is monitored from front side of the screen.
2. Description Related to the Prior Art
As disclosed in Japanese Patent No. 3257067 and Japanese Patent No. 3339504, the liquid-crystal projector is well known as an image displaying apparatus, which displays the images on the screen by projecting light modulated by liquid-crystal displays to the screen. As types of liquid-crystal projector, there are a front type in which the images are projected from the front side of the screen, and a rear type in which the images are projected from the rear side of the screen. In addition, the liquid-crystal projectors are categorized into a transmissive type and a reflective type, according to type of the liquid-crystal display used in the liquid-crystal projector.
Illuminating light from a light source illuminates the liquid-crystal displays after condensed and formed to approximately parallel rays by an illuminating optical system. If an illuminating area is too large to a size of the liquid-crystal displays, usability of the illuminating light becomes down. In addition, if the illuminating area is too small to the size of the liquid-crystal displays, quality of the image displayed on the screen becomes down.
In an image forming optical system, for example, three liquid-crystal displays corresponding to blue, green, and red color light are provided. The illuminating light radiated from the illuminating optical system is decomposed into three-primary-colors channels including blue, green, and red color light that illuminates the corresponding liquid-crystal display. And three-primary-colors lights, modulated by the corresponding liquid-crystal display by each color channel, are projected on the screen by a projection lens after being composed or synthesized by for example a composite prism.
In the liquid-crystal projector, for example an ultra-high pressure mercury lamp or a metal halide lamp is used as the light source. Performance of the liquid-crystal projector largely depends on performance of the light source. If a powerful light source is used, it will become possible to project images to a bigger screen, and to project clear images even if it is a bright place. However, if the powerful light source is used, manufacture cost will increase and the price of a product will become high. Therefore, it is necessary to choose the light source used for the liquid-crystal projector according to a user's needs.
Since the user's needs are various, the user often requests using a specific light source. In this case, a change of the light source is made. According to a new light source, the illuminating optical system must also be modified so that the illuminating light from the new light source may illuminate the suitable range that is slightly larger to the size of the liquid-crystal display.
However, in the conventional liquid-crystal projector, the illuminating optical system and the image forming optical system were unitized and contained in a same case. Therefore, in case of modification of the illuminating optical system accompanying change of the light source, since the case must also be changed, there is a problem that modification of the illuminating optical system accompanying change of the light source cannot be made easily.
Moreover, since the light source emits high heat, it is necessary to contain the illuminating optical system, arranged near the light source, in a case excellent in heat resistance. On the other hand, a case, which contains the image forming optical system that is arranged away from the light source, does not need great heat resistance. However, in case of the illuminating optical system and the image forming optical system are unitized, the image forming optical system is also contained in the case formed with the material excellent in heat resistance, according to the need of the illuminating optical system. Since the material excellent in heat resistance is expensive, there is the problem that the manufacture cost of the liquid-crystal projector becomes high.